Continuous flow centrifuge with means for reducing pressure drop

ABSTRACT

An intermediate speed, continuous flow, high capacity liquid centrifuge assembly characterized by a low pressure drop in the process liquid is described. The elongated, vertically oriented rotor contains a central, generally cylindrical core having radially projecting vanes defining a segmented annular cavity between the core and rotor wall. Hollow shafts support the rotor while providing axial inlet and outlet passageways for continuous flow of process liquid through the rotor during operation. Improved design of flow passages from the axial inlet passageway to respective sectors in the segmented annular cavity and from those sectors to the axial outlet passageway provides a significant reduction in pressure drops through the centrifuge.

lJnited States Patent 91 Clio 1 1 CONTINUOUS FLOW CENTRIFUGE WITH MEANS FOR REDUCING PRESSURE DROP [75] lnventor: Nakwon Cho, Knoxville, Tenn.

[73] Assignee: The United States of America as represented by the United States Atomic Energy Commission [22] Filed: May 25, 1971 [21] Appl. No.: 146,758

[4 1 May 1,1973

Primary Examiner-George H. Krizmanich Attorney-Roland A. Anderson [57] ABSTRACT An intermediate speed, continuous flow, high capacity liquid centrifuge assembly characterized by a low pressure drop in the process liquid is described. The elongated, vertically oriented rotor contains a central, generally cylindrical core having radially projecting vanes defining a segmented annular cavity between the core and rotor wall. Hollow shafts support the rotor while providing axial inlet and outlet passageways for continuous flow of process liquid through the rotor during operation. Improved design of flow passages from the axial inlet passageway to respective sectors in the segmented annular cavity and from those sectors to the axial outlet passageway provides a significant reduction in pressure drops through the centrifuge.

8 Claims, 5 Drawing Figures PATENTED H975 3,730,422

sum 1 0F 2 ATTORNEY.

PATENTED H975 3,730,422

SHEET 2 UF 2 PRIOR PRESSURE DROP O PSI 4- o R 0 IMPROVED MACHINE O 5 1O 2O 3O OPERATING SPEED RPMMO' INVENTOR.

N a k we n C h 0 ATTORNEY.

CONTINUOUS FLOW CENTRIFUGE WITH MEANS FOR REDUCING PRESSURE DROP BACKGROUND OF THE INVENTION The invention described herein relates generally to liquid centrifuges and more specifically to the improved design of flow passages characterized by low pressure drops for use in liquid centrifuges. It was made in the course of, or under, a contract with the U. S. Atomic Energy Commission.

Liquid zonal centrifuges designed for the large scale isolation or separation of virus for vaccine production have gained extensive use within the past several years. A widely used liquid centrifuge of the zonal type which was especially designed for separating virus is described in U. S. Pat. No. 3,430,849, issued Mar. 4, 1969, to common assignee in the names of Ronald F. Gibson, Clifford E. Nunley, and Dean A. Waters. A problem well known in the prior art and associated with the above-described type of liquid centrifuge involved a speed dependent pressure drop which limited throughput of process liquid both from the standpoint of instantaneous flow rates and total operating times between cleanouts, created sealing problems due to the higher process liquid pressures necessary to operate the machine, and caused undesirable shifting and compression of density gradient bands with a consequential loss of product purity. Pressure drops exceeding 15 pounds per square inch were typical at normal operating speeds of 35,000 rpm in the aforementioned centrifuge.

It is, accordingly, a general object of the invention to provide a continuous flow liquid centrifuge characterized by low pressure drops in the process liquid at operating speed.

Other objects of the invention will be apparent to those skilled in the art upon examination of the following description of the preferred embodiment and appended drawings.

SUMMARY OF THE INVENTION In a continuous flow, high capacity liquid centrifuge wherein an elongated rotor contains a central core defining a peripheral annular cavity axially segmented into a plurality of sectors, axially extending inlet and outlet flow passageways at opposite axial extremities of the rotor, and a plurality of connecting passages communicating, respectively, between the inlet and outlet passageways and the plurality of sectors, the improvement comprising the connecting passages being sized so as not to enlarge the effective cross-sectional area of the inlet and outlet passageways where they communicate with those passageways. To prevent such enlargement, the connecting passages must be sized so as to terminate in a common plenum no larger than the axially extending passageways. In addition, the connecting passages should be sized so that the sum of their cross-sectional areas is at least as great as that of the axially extending inlet or outlet passageway which they communicate with. Centrifuges made in accordance with the invention exhibit greatly reduced pressure drops which facilitate increased process liquid throughput, reduce sealing problems, and result in more stabilized gradient distributions where zonal centrifugation is practiced.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a vertical section view of a liquid centrifuge rotor incorporating the improvement of the present invention.

FIG. 2 is a partial horizontal section view of the centrifuge rotor of FIG. 1.

FIG. 3 is an enlarged, vertical section view of a core cap insert used in the centrifuge of FIGS. 1 and 2.

FIG. 4 is a top plan view of the core cap insert of FIG. 3.

FIG. 5 is a graph comparing the speed dependent pressure drops developed in a centrifuge made in accordance with the prior art and one incorporating the improvement of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring initially to FIGS. 1 and 2, detailed vertical and horizontal section views of a centrifuge rotor assembly made in accordance with the invention are shown. As shown, rotor bowl I is closed at its top and bottom ends by rotor end caps 2 and 3. Disposed within rotor bowl 1 is a hollow core insert 4 threadably engaged by core end caps 5 and 6, each of which is formed with a stepped central cylindrical cavity 7 and six radially extending grooves 8. Hollow core insert 4 is provided with six radially projecting, axially extending septa or vanes 9 which divide the sample volume into axially extending sectors 11. As indicated in FIG. 2, grooves 8 communicate between respective sectors 11 and the stepped central cavity 7 within each core end cap.

Each of rotor end caps 2 and 3 is provided with an axially extending central passageway 12 which terminates within the rotor at the apex ofa conical depression 13. The other end of each passageway 12 is in communication with passageways extending through respective upper and lower rotor shafts (not shown) which drive and provide support to the centrifuge rotor assembly. Additional details relating to the rotor shafts, drive, and ancillary systems used with the rotor assembly can be found in U. S. Pat. No. 3,430,849.

According to the invention, axially extending passageways 12 are joined to radially extending grooves 8 by means of short, inclined flow channels 14. Channels 14 are conveniently formed by grooving the conical end surface 15 of each insert 16 as shown in greater detail in the enlarged vertical section and plan views of FIGS. 3 and 4. Conical end surface 15 is designed to mate with matching conical depression 13 in rotor end caps 2 and 3. Alignment of channels 14 with grooves 8 is accomplished by means of alignment pins 17. Each conical surface 15 is urged into intimate contact with a respective conical depression 13 by springs 19.

In order to achieve the full benefits of the invention and minimize pressure drops in the process liquid, channels 14 must be sized to avoid flow restriction by either reduction of flow area or the development of vortex flow in the process liquid. The most critical point where reduction of flow area or vortex flow is likely to develop is in the plenum formed by the intersection ofchannels 14 at the apex of conical surface 15. That plenum serves as an inlet plenum for channels 14 where passageway 12 is an inlet flow passageway and as an outlet plenum where passageway 12 is an outlet flow passageway. Reduction of flow area is avoided by making the sum of the flow cross-sectional areas of channels 14 within each surface 15 at least as large as the flow cross-sectional area of a corresponding passageway 12. Vortex flow is avoided by limiting the central opening or diameter d of the plenum corresponding formed at the apex of surface 15 to the diameter D" of corresponding passageway 12. In order to satisfy both requirements, the depth of channels 14 will normally be greater than their width, which has an upper limit set by the vortex flow limitation. A very close approximation of the maximum permissible width of channels 14, assuming the channels are of equal widths, may be obtained from the relationship:

mur

where w is the maximum width of each channel C is the circumference of passageway 12; and

n is the number of channels 14 in a given surface 15. Where the sum of the widths of channels 14 exceeds the circumference of passageway 12, d is made larger than D and speed dependent vortex flow develops with a resultant large pressure drop across the vortex.

As shown, the depth and width of channels 14 increase at increasing distances from the apex of surface 15. Channels 14 of uniform depth and/or width may be used without departing from the scope of the invention, although a slightly greater pressure drop will occur along the channels.

To ensure a minimum pressure drop in the apex region of inserts 16, channels 14 should be of maximum permissible width so that d equals D". The channels will then intersect in the manner shown in FIG. 4 with the segmented conical face portion 17 each terminating in a sharp point in the apex region of surface 15. In addition, the secondary apex 18 formed by the intersecting channels 14 should come to a sharp point as shown in FIG. 3. Such configuration represents the optimum shape for inserts 16 since it maximizes the use of available flow area without permitting vortex flow to develop, provides uniform flow distribution to the various channels 14, and substantially eliminates pressure drops caused by the presence of stagnation points.

It is also noted that the innermost wall of channels 14 formed by conical depression 13 are inclined at an angle a of about 135 from the axis of passageway 12. That inclination has been found to provide the minimum pressure drops when used in connection with the aforementioned vortex flow and flow restriction requirements.

In some applications it may be desirable to increase the diameter of passageways 12 to increase its flow capacity. In that case vortex flow may develop along the length of the passageway. The particular diameter where vortex flow will develop varies with the centrifuge operating speed and cannot always be predicted, although applicant has determined that passageways 12 up to 3/l 6 inch in diameter can be used without serious vortex problems. In those applications, axially extending septa or vanes, or fluted walls, may be used to divide the passageway and thereby prevent vortex flow from developing. The present invention can be employed in combination with the septa or fluted walls in the larger passageways. In that case, care must be taken to extend the septa or other anti-vortex devices the entire way into the apex of inserts 16 and the septa should be positioned to provide equal flow distribution to each of channels 14.

Referring now to FIG. 5, the results of pressure drop tests are shown by means of a graph comparing pressure drop across centrifuge assemblies at various operating speeds. The curve labeled prior art represents the speed dependent pressure drops in centrifuge assemblies made in accordance with the teachings of U. S. Pat. No. 3,430,849. At 35,000 revolutions per minute, a pressure drop of 18.5 pounds per square inch was experienced. This point is not shown on the graph of FIG. 5, but the steep upward trend of the prior art pressure drop curve can be seen within the limits of the scale. Using an otherwise identical centrifuge but with improvements described herein, pressure drops of slightly less than 4 pounds per square inch were measured, less than one-fourth of the prior art pressure drop. In both machines, passageways 12 were Vs inch in diameter and six radially extending grooves 8 were used. In the improved machine, the width of each channel 14 at the apex of surface 15 was l/32 inch and its depth 1/16 inch.

Although the invention has been illustrated as applied to a specific liquid centrifuge system, it will be apparent to those skilled in the art that the invention can be beneficially applied to a wide variety of continuous flow liquid centrifuges with a minimum of adaptation or experimentation. Many alternative devices other than the particular inserts 16 illustrated could be used which satisfy the limitations on vortex flow and flow area restrictions stated herein. It is, accordingly, intended that the invention be limited only by the scope of the appended claims.

What is claimed is:

1. In a continuous flow liquid centrifuge wherein an elongated vertically oriented rotor contains a central core defining a peripheral annular cavity axially segmented into a plurality of sectors, axially extending inlet and outlet flow passageways at opposite axial extremities of said rotor, respective inlet and outlet plenums at the inner ends of said inlet and outlet flow passageways, and a plurality of connecting passages communicating, respectively, between said inlet and outlet plenums and said sectors; the improvement characterized by said connecting passages being sized so that the sum of their flow cross-sectional areas is at least as great as said inlet and outlet flow passageways, said inlet and outlet plenums being no larger diametrically than said inlet and outlet passageways so as to avoid vortex flow conditions therein.

2. The improvement ofclaim 1 wherein said inlet and outlet plenums are the same size diametrically as said axially extending inlet and outlet flow passageways.

3. The improvement of claim 1 wherein said connecting passages are inclined at an angle of about with respect to the axes of said inlet and outlet flow passageways at their ends in communication with said inlet and outlet plenums.

4. The improvement of claim 1 wherein said connecting passages increase in cross section at increasing distances from said plenums along at least part of their lengths beginning at said plenums.

5. In a continuous flow liquid centrifuge comprising an elongated vertically oriented rotor, upper and lower rotor end caps closing the upper and lower ends of said rotor, axially extending inlet and outlet passageways passing through the central hub portions of said rotor end caps, and a central core assembly disposed within said rotor, said core assembly defining a peripheral annular cavity axially segmented into a plurality of sectors, radially extending grooves communicating at their peripheral ends with said sectors being provided in the upper and lower ends of said core assembly; the improvement comprising: a central conical depression being provided in each of said rotor end caps and having an apex centered to register with said inlet and outlet passageways in said end caps, and a removable insert centrally disposed within the upper and lower ends of said core assembly with a conical end surface for mating with said conical depressions in said rotor end caps, said end surface of each of said inserts being pro vided with a plurality of grooves extending from the apex to the base of said conical end surface, said grooves in said end surface being angularly spaced so as to register with said radially extending grooves in said core assembly so as to provide a continuous flow passage from said axially extending inlet and outlet passageways to each of said sectors, said plurality of grooves in said end surface forming a plenum at the apex of said insert which is no larger diametrically than said inlet and outlet passageways, the sum of the crosssectional areas of said grooves where they form said plenum being at least as great as the cross-sectional area of said inlet and outlet passageways.

6. The improvement of claim 5 wherein said inserts are spring loaded within said core assembly so as to be urged into close contact with said conical depressions in said rotor end caps.

7. The improvement of claim 5 wherein the slant angle of said conical end face is about 45.

8. The improvement of claim 5 wherein said insert is keyed to said core assembly to maintain alignment of said grooves in each of said end surfaces with said radially extending grooves in the upper and lower ends of said core assembly. 

1. In a continuous flow liquid centrifuge wherein an elongated vertically oriented rotor contains a central core defining a peripheral annular cavity axially segmented into a plurality of sectors, axially extending inlet and outlet flow passageways at opposite axial extremities of said rotor, respective inlet and outlet plenums at the inner ends of said inlet and outlet flow passageways, and a plurality of connecting passages communicating, respectively, between said inlet and outlet plenums and said sectors; the improvement characterized by said connecting passages being sized so that the sum of their flow cross-sectional areas is at least as great as said inlet and outlet flow passageways, said inlet and outlet plenums being no larger diametrically than said inlet and outlet passageways so as to avoid vortex flow conditions therein.
 2. The improvement of claim 1 wherein said inlet and outlet plenums are the same size diametrically as said axially extending inlet and outlet flow passageways.
 3. The improvement of claim 1 wherein said connecting passages are inclined at an angle of about 135* with respect to the axes of said inlet and outlet flow passageways at their ends in communication with said inlet and outlet plenums.
 4. The improvement of claim 1 wherein said connecting passages increase in cross section at increasing distances from said plenums along at least part of their lengths beginning at said plenums.
 5. In a continuous flow liquid centrifuge comprising an elongated vertically oriented rotor, upper and lower rotor end caps closing the upper and lower ends of said rotor, axially extending inlet and outlet passageways passing through the central hub portions of said rotor end caps, and a central core assembly disposed within said rotor, said core assembly defining a peripheral annular cavity axially segmented into a plurality of sectors, radially extending grooves communicating at their peripheral ends with said sectors being provided in the upper and lower ends of said core assembly; the improvement comprising: a central conical depression being provided in each of said rotor end caps and having an apex centered to register with said inlet and outlet passageways in said end caps, and a removable insert centrally disposed within the upper and lower ends of said core assembly with a conical end surface for mating with said conical depressions in said rotor end caps, said end surface of each of said inserts being provided with a plurality of grooves extending from the apex to the base of said conical end surface, said grooves in said end surface being angularly spaced so as to register with said radially extending grooves in said core assembly so as to provide a continuous flow passage from said axially extending inlet and outlet passageways to each of said sectors, said plurality of grooves in said end surface forming a plenum at the apex of said insert which is no larger diametrically than said inlet and outlet passageways, the sum of the cross-sectional areas of said grooves where they fOrm said plenum being at least as great as the cross-sectional area of said inlet and outlet passageways.
 6. The improvement of claim 5 wherein said inserts are spring loaded within said core assembly so as to be urged into close contact with said conical depressions in said rotor end caps.
 7. The improvement of claim 5 wherein the slant angle of said conical end face is about 45*.
 8. The improvement of claim 5 wherein said insert is keyed to said core assembly to maintain alignment of said grooves in each of said end surfaces with said radially extending grooves in the upper and lower ends of said core assembly. 